Thin film transistors (TFTs) made in whole or in part of organic materials can be less expensive and easier to manufacture than traditional transistors and can be used in applications where traditional transistors are not economical and high density is not required. For example, organic thin film transistors could be used for electronic paper, posters and books, smart cards, toys, appliances and electronic bar codes for product identification. Organic TFTs can also be made from flexible materials, and such TFTs can be used to control diodes in flexible panel displays for computers, laptops and televisions.
A particular advantage of organic TFTs lies in the opportunities for manufacturing of TFTs using thermal printing methodologies that reduce or eliminate the need for chemical etchants, photomasks, and solvent-based processes. There are several known examples of thermal transfer of layers for the construction of electronic devices.
A particular need for printable electronics includes thermally imageable insulating layers or dielectric passivation layers. WO 2005/004205, for instance, discloses a method of forming a pattern of filled dielectric material on a substrate by a thermal transfer process comprising exposing to heat a thermally imageable donor element comprising a base film, a light to heat conversion (LTHC) layer, and a transfer layer of dielectric material. In a thin film transistor (TFT), the dielectric layer serves to insulate the gate electrode from the semiconductor and source-drain electrodes. Its primary function is to allow the passage of fields, but not currents. Fundamental requirements are that the dielectric layer possess high volume resistivity, greater than 1014 ohm-cm, to prevent leakage currents; and be largely pinhole free to prevent catastrophic shorts between conductive layers. The dielectric layer also must have high purity in order not to dope the adjacent semiconductor layer; it should be thin, for instance, about 5 microns or less, and have a high dielectric constant for low-voltage operation.
Another particular need for printable electronics are thermal transfer donors that allow patterned thermal transfer of conducting metal layers that exhibit good transfer properties and good adhesion to a variety of materials. For instance, WO 03/035279 A1 discloses precursor compositions and a method for the deposition of electronic features including conducting layers. Particularly desirable are thermal transfer donors wherein, after transfer, no further heating or firing of the patterned metal layer is required to achieve a high conductivity metal layer.
Needed are TFTs comprising a dielectric layer and/or electrodes that can be provided by a dry thermal transfer process. For optimal performance, the TFT and TFT arrays should have Ion/off, or median Ion/off in the case of the TFT array, of preferably about 1E+05 or greater and more preferably about 1E+06 or greater. Additionally, the performance of the TFT arrays should be characterized by very high yields of functioning devices, preferably about 98% or greater, more preferably about 99% or greater and most preferably, about 100%. Also desirable are a long TFT lifetime, preferably a drop in Ion/off after 50 days about half the original value or less; and a high mobility, preferably about 0.05 cm2V−1s−1 or greater, more preferably about 0.1 cm2V−1s−1 or greater and most preferably about 0.5 cm2V−1s−1 or greater. Desirable are TFT arrays with narrow distributions of threshold voltage, preferably about 85% with a threshold voltage distribution of about 10 V or less; and TFT/capacitor arrays with yields of both TFT's and capacitors of about 98% or greater, more preferably about 99% or greater and most preferably about 100%.